The present invention relates to an improved method of dyeing plastic articles such as sheets, and particularly those having an abrasion-resistant hard surface coating thereon. The invention also relates to a novel dye bath composition to be used in said dyeing process.
This invention is particularly useful for dip-dyeing polarized plastic sheets having a hard coat thereon to be used, for instance, as sunglass lenses, and hence the description of the invention will be given below mainly in conjunction with such an application, but the invention is not limited thereto. Thus, uncoated CR-39 (allyl diglycol carbonate) lens blanks can also be dyed successfully by use of the dye solution of this invention, preferably at a temperature higher than those suitable for hard-coated polarized lenses.
The most common dye bath system used for dyeing polarized lenses consists primarily of a dye or dyes dispersed in water with a minor amount of organic solvent, or a mixture of organic solvents added thereto. This type of dye bath system, however, has the disadvantage that water has a marked tendency to cause hazing of the lens. Because of these disadvantages, the primarily-aqueous dye bath system is not suitable for dyeing hard-coated polarized lenses to deep shades; they can be dyed only to a light tint.
Solvents may be added to speed up the aqueous dyeing process or to permit deeper shades. However, by their nature solvents attack the surface of the lens and cause deterioration of the surface hard coat and/or the substrate (crazing, pitting, swelling, etc.).